Problems of coherence and effectiveness have been repeatedly mentioned by external observers as affecting European Union (EU) external action. A theory-based explanation of this state of affairs is, however, lacking in the literature. This thesis aims to address this lacuna by focusing on a particular aspect of EU external action, the Common Foreign and Security Policy (CFSP), and by analysing CFSP actions in Bosnia and Herzegovina (BiH) throughout the period 1991-2006. Its primary goal is to determine which factors explain different degrees of coherence and effectiveness, and, in particular, whether or not, increasing CFSP institutionalisation has promoted coherence and effectiveness. After introducing the concepts of coherence and effectiveness, the thesis sketches the political and academic debate surrounding these concepts, and three key explanatory factors are singled out: interests, identities and institutions. The latter is taken as a starting point for analysis. Drawing on a historical institutionalist theory, it explores the CFSP institutionalisation and examines how the development of institutions, path dependency, unintended consequences , learning and socialisation have affected CFSP coherence and effectiveness over time. Furthermore, it aims to find out whether changes in institutions have facilitated convergence of Member States' interests regarding EU policy towards BiH, as well as the development of the EU's broader international role. The thesis then proceeds to investigate CFSP activities in BiH, analysing coherence and effectiveness in eight case studies. It concludes that CFSP institutionalisation has partially increased levels of effectiveness over the period 1991-2006. As for coherence, greater institutionalisation has resulted in some problems of coherence in the short term, but leaming has been a significant factor correcting these in the medium and long term. While path dependency has resulted in problems of coherence and effectiveness, socialisation processes have been important in facilitating consensus among the Member States. Finally, CFSP institutionalisation is seen as having had an impact on the EU's international role moving it from a civilian to a normative power, willing to resort to military instruments when necessary.